The present invention relates to toners and developers for electrophotography to be used in developing electrical or magnetic latent images in electrophotography, electrostatic printing, and the like.
Toners for developing electrical or magnetic latent images and the like are used in various processes for forming and recording images. One of such image forming processes is electrophotography, which uses a photosensitive member generally formed of a photoconductive material, and wherein an electrical latent image is formed on the photosensitive member by various means. The electrical latent image is developed using a toner. The toner image thus developed is transferred to a recording material, such as paper, and then fixed thereto by heating and/or pressure, or by using solvent vapor or the like, thus obtaining a copy of the image. Where a process for transferring toner images to a recording material is included, there is usually also provided a process for removing the toner remaining on the photosensitive member.
The following are examples of developers conventionally used in dry development devices for electrophotography:
1) One-component-type magnetic developers comprising a toner containing magnetic powder.
2) One-component-type non-magnetic developers comprising a toner containing no magnetic powder.
3) Two-component-type non-magnetic developers comprising a toner containing no magnetic powder and a magnetic carrier, which is mixed with the toner in a fixed proportion.
4) Two-component-type magnetic developers comprising a toner containing magnetic powder and a magnetic carrier, which is mixed with the toner in a fixed proportion.
Various development methods using such toners have been proposed and put into practical use. The toners used in these development methods are generally manufactured by a pulverizing method in which a coloring agent like a dye or pigment is mixed with, and uniformly dispersed in, a thermoplastic resin serving as the binder. The mixed substance thus obtained is then finely pulverized and classified to provide a desired particle size distribution. Toners typically contain a principal resin or toner resin, colorant and various functional additives such as release agents and charge control additives.
Heterogeneity of the toner particles"" composition is believed to be the root cause of numerous problems throughout the serviceable life of toner in a printing device. The print quality black-on-white (BOW) defect involving unwanted toner black spots on the printed product has been a recurring and sometimes serious problem in certain commercial printers over the past few years. The black spots are highly visible in the background region of the print and are non-repeating in nature. They can and do occur anywhere in the background region of the print and usually appear later in the cartridge life. The onset of this print quality defect depends upon many factors including the print job length. As the print job length decreases (e.g., from 4 to single page jobs), the severity of the black spots increases, and the number of prints before the onset of the defect decreases (e.g., from 16,000 to 10,000 prints). Furthermore, it appears that the extent of these spots and the location of their onset with respect to the number of pages printed is also a function of the percent coverage of toner on the page. Another factor which has proven to affect the level of this defect is the number of developer roll revolutions since the beginning of the cartridge life. The higher the number of such revolutions, the quicker the onset of the spots and the greater their extent.
Even though the precise mechanism for the formation of such black-on-white defects is unknown, it has been observed that a number of factors associated with the toner affect the extent of these defects and the time it takes for their onset. Various extra-particulate additives (EPAs) have been found to reduce the number of spots and delay their onset to higher print counts. However, increasing the level of such additives does not totally eliminate the spots, but does contaminate the charge roll and other machine parts, thereby shortening their useful life. A concern with EPAs in general is the increased abrasivity of the resulting toner and the resulting increase in wear on the contacting device components. Nevertheless, to date no EPAs have been found to totally and consistently eliminate these BOW defects without causing premature cartridge failure.
While Applicants do not wish to be bound by any theory of the mechanism of BOW defect formation, it is presently believed that the toner particle compositional uniformity is a major factor in the existence of such defects. Heterogeneity of the toner particles"" composition is believed to be the root cause of observed selectivity throughout the life of the cartridge. An experiment was conducted to prove that the compositional make-up of the toner left in the developer station after the onset of BOW defects is the primary factor affecting this phenomenon. After the onset of black spots was observed in a printer with a particular electrophotographic cartridge, xe2x80x9cfreshxe2x80x9d toner was added to the developer station and the spots disappeared for a few thousand pages. However, they did return. Moreover, when xe2x80x9cusedxe2x80x9d end-of-life toner is added to a new cartridge, the defects begin almost immediately. Despite these observations, it has yet to be analytically determined what differences in the toner from beginning of cartridge life to the onset of BOW defects are contributing to the formation of these print quality defects.
The present invention successfully eliminates the lack of heterogeneity in toners with the addition of two functional additives reactive with each other to form a stable reaction product. The copolymer reaction product apparently acts as a compatibilizer to improve the dispersion of various polymeric components (such as a release agent) with the backbone structure of the toner resin.
Most toner compositions employ release agents such as waxes and/or silicone polymers. Poly(dimethysiloxane) resins or oils (PDMS) are well known to exhibit excellent external release agent characteristics (i.e. when applied to fuser rolls) due to their extremely low surface energy. The property is highly desirable in contact-fusing electrophotography because it is important to be able to release the toner from the hot-oiled fuser roll and thus prevent hot offset from occurring. Several different low molecular weight organic materials have been used in the toner industry to try to eliminate this hot offset phenomenon. Low molecular weight polyolefin waxes are by far the most common type of internal release agent. Each type of release agent has its own advantages and disadvantages. For example, polyolefins tend to crystallize to a significant extent (between 70 and 90 volume percent crystallinity). When these molecules crystallize, they phase segregate from the toner resin and form large wax domains which cause numerous print quality defects, as well as a wax imbalance between the toner fines and the average sized toner particles. Poor homogeneity of these additives in the toner particles tends to cause a number of problems, the most important of which are low toner powder flow and variations in triboelectric charge distribution, which can lead to print quality defects.
Methods have been reported for compatibilization of immiscible blends of polymers by reactive mixing, in which functionalized versions of the polymeric components react in situ to form a block copolymer compatibilizer. The fundamental requirements for such reactive processing include the following: There must be sufficient mixing to achieve the desired fineness of morphological texture. Some of the polymer molecules must contain chemical functional groups which can react to form primary bonds during the mixing/mastication process. The functional groups must be of sufficient reactivity for reactions to occur across melt-phase boundaries. The reactions must occur rapidly enough to be completed during processing in the extruder or mixer within a reasonable time. The bonds formed as a result of reactive blending must be stable enough to survive subsequent processing. In short, the compatibilization reactions should be fast and irreversible.
The use of blends and alloys of immiscible polymers has increased because it is generally less expensive to develop a new blend composition than to develop new polymers based upon new monomers to meet needs for specialized types of polymers. Compatibilizing methods and agents are required for such blends, since most polymers are mutually immiscible and have poor interfacial adhesion. Compatibilizers generally are believed to act at interfaces to improve interfacial interactions between immiscible polymeric species. Thermodynamic miscibility or compatibility is discussed briefly by Bonner and Hope, infra. It is recognized that miscibility between polymers is determined by a balance of enthalpic and entropic contributions to the free energy of mixing. While for small molecules the energy is high enough to ensure miscibility, for polymers the entropy is almost zero, causing enthalpy to be decisive in determining miscibility. The change in free energy on mixing (AG) is written as
xcex94G=AHxe2x88x92Txcex94S
where H is enthalpy, S is entropy and T is temperature (K). For spontaneous mixing, xcex94G must be negative, and so
xcex94Hxe2x88x92Txcex94S less than 0.
This implies that exothermic mixtures (xcex94H less than 0) will mix spontaneously, whereas for endothermic mixtures miscibility will only occur at high temperatures. However, thermodynamic compatibility need not be attainedxe2x80x94technological compatibility, where the blend has useful properties, normally is sufficient. Mechanical or chemical techniques can be used to attain technological compatibility.
Technological compatibility of immiscible polymers can be produced by: the addition of a compatibilizer before or during the mixing/blending process; adjustment of viscosity ratios to favor rapid formation of the desired phase morphology during mixing; in situ formation of a compatibilizer during the mixing/blending process; and introduction of crosslinks in one of the phases. The objectives of technological compatibilization are to produce compositions which exhibit good ultimate properties, e.g. strength, elongation, fatigue life, etc. Compatibilized polymer blends exhibit at least some of the following differences from uncompatibilized polymer blends: Reduced morphological dimensions (smaller domain sizes, thus smaller potential flaws); Improved bonding or adhesion between phases; and reduced tendencies to form highly shaped domains during flow in processing, molding, etc.
One approach to technological compatibilization is the addition of a compatibilizer before or during the mixing/blending process. Such compatibilizers are frequently a block copolymer. To be effective, the compatibilizing block copolymer must possess segments with chemical structures or solubility parameters, which are similar to or the same as those of the polymers being blended, and a sufficient amount of the compatibilizing polymer must be located at the interface of the polymer phases. Such copolymer compatibilizers are disclosed in numerous patents assigned to Xerox Corporation, e.g. U.S. Pat. No. 5,229,242. A method of promoting the presence of a compatibilizing block copolymer at the interfacial region is to use reactive mixing techniques, whereby the compatibilizing copolymer forms at the interface. In such cases, polymer molecules of one phase contain functional groups which chemically interact with molecules of a polymer in an adjacent phase, so that a compatibilizer forms in the interfacial regions where it is needed.
Bonner and Hope provide an extensive discussion of compatibilization of polymer blends in xe2x80x9cCompatibilization and reactive blending,xe2x80x9d Chapter 3 of Polymer Blends and Alloys, Edited by M. J. Folkes and P. S. Hope, Blackie A and P, 1993.
U.S. Pat. No. 5,310,616 to Akamatsu discloses toner compositions for electrostatic copiers containing siloxane resins with difunctional and trifunctional siloxane units as charge regulating agents. The siloxane units of the resins can contain aminofunctional hydrocarbon radicals or haloalkyl radicals.
U.S. Pat. No. 4,876,169 to Gruber et al. discloses toner compositions containing particles of polyesters with siloxane block segments, the particles containing release additives.
U.S. Pat. No. 5,202,215 to Kanakura et al. discloses toners which are prepared by dispersion-polymerizing a vinyl monomer in the presence of a dispersion polymerization stabilizer, a polymerization initiator and silicone-containing organic resin microparticles in a dispersion medium which dissolves the vinyl monomer, the polymerization stabilizer and the initiator but not the silicone-containing organic resin microparticles or the resulting toner particles. Amino-silicone compounds can be used in the microparticles.
U.S. Pat. No. 5,059,505 to Kashihara et al. discloses spherical resin particles for electrophotographic toners, prepared by a process involving a dispersion stabilizer which can contain a dialkylaminosiloxane. The toner resin itself can contain monomers such as styrene and maleic acid.
U.S. Pat. No. 5,512,406 to Takeda et al. discloses toner compositions having bimodal particle size distributions, produced by admixing toner portions having different particle size distributions with functional additives, then combining the portions thus prepared.
One advantage of the present invention includes successfully reducing or eliminating the disadvantages that come with using siloxane polymer materials as internal release agents in dry electrophotographic toners by incorporating a compatibilizer (as mentioned above) which prevents the low surface free energy molecules from migrating to the toner surface and thus causing phase separation from the toner resin. The present invention is also successful in compatibilizing toner resins with a variety of other polymeric components of toner compositions, including colorants, wax release agents and charge control agents.
The main aspect of the present invention is the reactive compatibilization of toner resins with other polymeric components of the toner composition.
Other aspects of this invention will appear from the following description and appended claims, reference being made to the accompanying drawing forming a part of this specification wherein like reference characters designate corresponding parts.
In accordance with the present invention, various aspects and advantages are achieved by the employment of a reactive compatibilizer in toner compositions to compatibilize the toner resin with other polymeric components such as release agents, colorants or charge control additives. Broadly, the improved toners of the invention comprise a primary resin (i.e., toner resin) and at least one additional polymeric component, plus effective amounts of a functional secondary resin and a functional polymer reactive therewith to form a stable reaction product having monomer components compatible with at least one monomer component of the primary resin and of at least one of the additional polymeric component(s), the reaction product being present in an amount effective to compatibilize the primary resin and the additional polymeric component(s). For instance, the additional components compatibilized with the toner resin can be additional modifying resins, release agents such as silicone polymers or waxes, and/or charge control additives such as carboxylated (sulfonated) polystyrenes and calix(n)arene compounds. The additional polymeric components(s) to be compatibilized can also include colorants such as organic polymeric dyes, polymer-grafted carbon black and the like.
In preferred embodiments, the toner compositions comprise the primary resin, colorant and a first silicone polymer as release agent, plus effective amounts of a functional secondary resin and a functional silicone polymer reactive therewith to form a stable reaction product effective as a compatibilizer, this reaction product being present in the finished toner composition. For instance, in the examples below the additives used were a styrene-maleic anhydride copolymer and an amino-functional silicone oil. These materials reacted to form a styrene-silicone oil copolymer amic acid, represented generically by formula III of the sole FIGURE. Such stable reaction products can be produced by a number of combinations of copolymers based upon aromatic vinyl monomers and dibasic acid anhydride monomers reacted with a functional silicone polymer. A novel family of such aryl-silicone copolymer amic acids are claimed herein.
A further embodiment of the invention includes processes of preparing the improved toner compositions of the invention, comprising steps of:
a) combining materials comprising a primary resin and a first silicone polymer or other release agent, plus effective amounts of a functional secondary resin and a functional silicone polymer reactive therewith to form a stable reaction product effective as a compatibilizer,
b) extruding the ingredients under conditions effective to form the reaction product of the secondary resin and the functional silicone polymer, and
c) grinding the extruded and hardened product.